Essentially it's just vanilla WoW with some extra rules to make it more difficult / punishing:

• Ironman. Your character has one life, and if they die, you delete them or you can keep them around as a ghost.
• You cannot use the auction house.
• You cannot trade with other players.
• You can group with players to do an instance. Each instance can only be done once, whether you succeed in finishing it or not.
• You cannot group with other players under any other circumstance.
• You cannot ask other players for help or buffs, but you and others are free to buff / help each other of your own volition.

The real fun is joining a hardcore (HC) guild, which is linked up to the other HC guilds on your faction with a universal guild chat. You can talk to other HC players, ask questions, brag about a cool BoE you found, and most importantly, get access to death announcements.

Any time someone in one of the guilds dies, a big raid warning pop-up springs up on the screen with a eulogy in guild chat, announcing the dead character's name, class, level, zone they died in, enemy that killed them, and their final words (which seems to pull from guild, party, say, or yell, whichever they used last).

I'm extremely tempted to make a masterpost or two of eulogies, there's some really great ones